Various types of thermoplastic elastomer compositions are known and have found use in tire and other industrial rubber applications, as well as in other manufactured products.
One such thermoplastic elastomer composition comprises a low-permeability thermoplastic resin, such as polyamide or a blend of polyamides, in which there is dispersed a low-permeability rubber, such as butyl rubber, halobutyl rubbers, or brominated isobutylene para-methylstyrene copolymers. The rubber is cured under conditions of dynamic vulcanization (curing the rubber during melt mixing as opposed to static curing that typically occurs in a rubber mold) and is intimately and uniformly dispersed as a particulate phase within a continuous phase of the thermoplastic resin. Because the rubber is cured under shear mixing conditions, these compositions are commonly known as dynamically vulcanized alloys (“DVAs”). Typically, the shear mixing for achieving the thermoplastic elastomer is accomplished by a mixer, such as multi-screw extruders.
Shear mixing of the resin and rubber allows the composition to possess near thermoelastic properties. This thermoelastic nature is desired for the flexibility, strength, and elongation desired for tire innerliners. Such dynamically vulcanized compositions allow for an air impermeability that is greater than that of a conventional bromobutyl inner liner. Due to the increase impermeability of the dynamically vulcanized composition, the gauge of the innerliner material may be reduced. However, even with the composition exhibiting thermoelastic properties, the resin component of the composition is less flexible than the elastomeric component, and as operating temperatures to which the material is subjected is decreased, this reduced flexibility may lead to reduced low temperature fatigue resistance and cracking of the composition.
EP 0 969 039 A1 discloses that small particle size rubber dispersed in a thermoplastic resin is important in order to achieve acceptable durability of the resulting composition, particularly where such compositions are intended to be used as innerliners in pneumatic tires. EP 0 969 039 A1 also discloses that, in attempting to fabricate a thermoplastic elastomer composition having sufficient flexibility, strength and elongation, as well as superior durability for use as an innerliner for pneumatic tires, by increasing the amount of rubber in the composition and keeping the ratio of melt viscosities of the rubber/resin at 1, the rubber becomes the principal matrix of the composition and the composition no longer exhibits thermoplasticity.
U.S. Patent Application Publication No. 2006/0293457 discloses a method for preparing a thermoplastic vulcanizate comprising: a) melt processing a blend of thermoplastic polymer and vulcanizable elastomer in a multi-screw extruder having at least three intermeshing screws; b) adding at least one curing agent to the melt-processed blend of a) in at least one location in the first 46% of the length of said extruder, or adding at least one curing agent to the melt-processed blend of a) in a second extruder, so as to initiate curing of the blend; and, curing at least partially said elastomer by reactive melt processing.
The manufacturing process by which the DVA is produced has a significant impact on the morphology of the DVA, and thus impacts the properties of the DVA. In particular, the DVA's low temperature durability is dependent not only on the composition but also on the morphology of the final product. Therefore, there is a continued need for improved processes for preparing DVAs that allow for similar or improved morphologies and properties of the DVAs than that which are currently achieved during conventional processes.